In this description and claims, the term “LED” will be used to denote both organic and inorganic LED's, and the invention can be applied to both categories. LEDs are current driven lighting units. They are driven using an LED driver which delivers a desired current to the LED.
The required current to be supplied varies for different lighting units, and for different configurations of lighting unit. The latest LED drivers are designed to have sufficient flexibility that they can be used for a wide range of different lighting units, and for a range of numbers of lighting units.
To enable this flexibility, it is known for the driver to operate within a so-called “operating window”. An operating window defines a relationship between the output voltage and output current than can be delivered by the driver. Providing the requirements of a particular lighting load fall within this operating window, the driver is able to be configured for use with that particular lighting load, giving the desired driver flexibility.
When an LED is driven to the desired current, the resulting voltage can vary in dependence on the characteristics of the LED itself. The operating window means that for each given current setting, there is a maximum voltage which can be supplied by the driver, before the limit of the permitted power supply is reached.
One of the degradation behaviours of an LED, in particular OLEDs, is the increase of the LED forward voltage over lifetime when driven at a constant current. As the current remains the same over the complete lifetime cycle, the increase of voltage creates an increase of power. The increase of power creates a higher temperature which in turn will increase the degradation of the LED even faster.
To prevent that the temperature of the LED becomes too high, the end-of-life (EOL) behaviour of the driver arranged is to switch off the output when the defined EOL LED voltage is reached.
A typical operating window of a window driver is shown in FIG. 1, which shows a region of permitted current and voltage values. For this arbitrary example, the LED driver can deliver any load current between 100 mA and 500 mA. There is an allowed voltage of 5 to 28 Volts and a maximum power of 10 Watt. The maximum power setting defines the curved part of the window boundary at the higher current and higher voltage regions, and the curve is of course defined by V(Volts)*I(Amps)<10.
FIG. 1 additionally shows the behaviour of a typical EOL solution when a 350 mA, 20 Volt OLED is operated over a long time period. The operating point moves over lifetime from point A, through B, C, D, E and F to point G. When the operating point reaches point G, the driver will switch off the OLED.
As mentioned above, the disadvantages of the current EOL implementation in particular for OLEDs are the increase of power, thus creating a higher temperature of the LED and with this increase of temperature, an accelerating degradation of the LED. This will faster increase the LED voltage, thus creating an even faster power increase. There is therefore an accelerated ageing process.
In the above example, the power over lifetime changes from 5.6 Watt at point A to 9.8 Watt at point G, which is nearly double the initial power.
FIG. 2 shows a plot over time of the electrical parameters (current, voltage and power output) of an LED when controlled using a constant current approach as shown in FIG. 1. The current remains constant to the end of life. The voltage and therefore power increase is not linear, but increases more rapidly over time as a result of the accelerated ageing caused by the increased heating as the power increases.
The constant current control is therefore not an optimum way to drive the LED if the lifetime is to be maximised.